After more than 40 years of searching, physicists at CERN have reached a major milestone in their hunt for one of the rarest known particle decays. Researchers working with the LHCb experiment have detected evidence of a process expected to occur only once in approximately 10 billion beauty particle decays.
The finding, announced during a seminar at CERN, marks the first time a single experiment has produced sufficient evidence for this extraordinarily rare event. Beyond its rarity, the decay is particularly valuable because it could help scientists investigate particles that are too massive to be created directly in the Large Hadron Collider.
At the center of the discovery is a particle called Bd, which contains a beauty quark and a down antiquark. Scientists found evidence that this particle can decay into a pair of muons (μ+μ–). Muons are similar to electrons but have greater mass. The results agree with predictions from the Standard Model, the leading theory describing fundamental particles and their interactions, and help narrow the possibilities for new particles beyond it.
Why Rare Particle Decays Could Reveal New Physics
One reason physicists are so interested in these unusual decays is that quantum effects allow other particles to influence how they occur. Even particles too heavy to be produced in the Large Hadron Collider can potentially leave subtle traces in the rates of these processes.
By measuring extremely rare decays with high precision, researchers can look for differences between experimental results and theoretical predictions. Such discrepancies could provide indirect evidence of previously unknown particles. Generally, the rarer a decay is, the more sensitive it can be to these hidden influences.
The decay known as Bd → μ+μ–belongs to a special category that physicists have long considered ‘golden channels’ for investigating physics beyond the Standard Model. During this process, a beauty quark and an antiquark with the opposite electric charge annihilate each other. Their energy is transformed into a muon and its antiparticle.
Scientists have previously observed a related process involving a different beauty particle, called Bs. In 2015, a joint analysis by the LHCb and CMS collaborations reported the first confirmed observation of Bs → μ+μ–, along with initial evidence for the much rarer Bd decay.
However, subsequent analyses incorporating data collected through 2018 could not confirm the earlier evidence for Bd → μ+μ–. The difference in rarity is substantial. The Bs decay occurs approximately three times per billion beauty particle decays, while the Bd process is predicted to happen about 30 times less frequently.
Upgraded CERN Detector Makes the Breakthrough Possible
A significant improvement to the LHCb experiment helped researchers overcome the enormous difficulty of detecting such a rare event.
Following a major upgrade, the experiment began collecting data in 2024 at five times the rate achieved between 2010 and 2018. The upgraded detector also became better at identifying different kinds of particles, giving researchers a clearer picture of the activity produced during collisions.
The team combined these improvements with machine learning techniques to distinguish the exceptionally rare signals from the vast number of other particle processes. Without these advances, the events scientists wanted to study would have been overwhelmed by the background.
“Physicists have been chasing this decay for more than forty years,” said Marco Santimaria, who co-led the international analysis team of 19 scientists. “Extreme precision is required to pick the muons out of the overwhelming background.”
Four Decades of Searching Finally Produce Strong Evidence
In particle physics, scientists use a statistical measure called standard deviations to evaluate how likely an apparent signal is to result from random fluctuations rather than a genuine physical process.
A result must reach three standard deviations to qualify as evidence of a new phenomenon. At that level, the probability of random fluctuations producing a signal at least as statistically extreme is approximately 0.3%.
Using data collected during 2024 and 2025, the LHCb researchers identified evidence of Bd → μ+μ– at a significance of 3.0 standard deviations. When they combined those measurements with earlier data gathered between 2011 and 2018, the statistical significance increased to 3.2 standard deviations, exceeding the widely accepted threshold for evidence.
The same investigation also produced the most precise measurements ever obtained for the related Bs → μ+μ–decay, providing additional information about these exceptionally uncommon processes.
Although the new findings represent a major achievement, physicists have not yet reached the threshold required to declare a confirmed observation. That milestone requires a significance of five standard deviations.
If future measurements achieve that level of certainty, Bd → μ+μ–would become the rarest beauty particle decay ever observed.
Future Large Hadron Collider Upgrades Could Uncover Hidden Particles
Researchers expect upcoming improvements to the Large Hadron Collider to make these measurements even more powerful.
The High Luminosity LHC, also known as the HiLumi LHC, is scheduled to begin operating in 2030. Another major upgrade to the LHCb experiment is planned for the middle of the following decade. Together, these developments could eventually increase the experiment’s data collection rate by another factor of ten.
Collecting much larger datasets would allow physicists to detect more of these extremely rare decays and measure their properties with greater precision. That increased sensitivity could make it possible to identify subtle effects associated with particles that remain beyond the direct reach of existing accelerators.
“Seeing evidence of this rare decay is incredibly exciting, opening up new possibilities to probe beyond the direct reach of the LHC,” explained Harry Cliff, LHCb’s Rare Decays Convener and Analysis Co-lead.
“The future LHCb upgrade will allow us to observe many more of these decays, providing even greater sensitivity to undiscovered particles.”
For physicists who have pursued this elusive signal for more than four decades, the new evidence represents a long-awaited advance. It also offers a promising way to investigate whether the known laws of particle physics are part of a much larger picture.
